The present invention relates to an arrangement of space vehicles in a constellation, wherein all spacecraft in the constellation operate to provide at least a minimum level of service. Additionally, at least one of the space vehicles in the constellation can act as a spare in the event of failure of any of the space vehicles in the constellation. The sparing arrangement is such that a minimum level of service is efficiently restored in the event of a spacecraft failure. The present invention also relates to a method for arranging space vehicles in a constellation and for replacing a failed space vehicle in a constellation.
A common approach for providing global communications coverage is to use multiple space vehicles (SVs) equally spaced in several inclined orbit planes. For example, one proposed constellation uses a Walker constellation arrangement that includes two inclined orbit planes, with 6 SVs in each plane (12 SVs total), and with the SV orbit ascending nodes separated by 180 degrees. Practical constellations include in-orbit spares to provide redundancy in the event that an operational SV fails. Typically, one or two non-operational spares per orbit plane are provided. The spares are stored in so called “sparing” orbits that have a different altitude and inclination from the mission orbit but precess at the same rate. If an SV in the constellation fails, a spare SV is transferred from the sparing orbit to the mission orbit to replace the failed SV. This approach is currently used for the Iridium, Globalstar, and the Intermediate Circular Orbit constellations.
One drawback of the prior-art sparing approach is that the spare SVs cannot be used to provide service. In effect, the spare SV resources, which may represent about 10 to 20% of the constellation, serve no useful benefit unless another SV fails. Additionally, the spare unused SVs degrade due to radiation exposure in the space environment, whether they are used or not, and ultimately become useless. Another drawback of the known sparing approach described above is that a failed SV cannot be rapidly replaced without a large fuel penalty. For example, for the proposed Walker constellation discussed above, for a failed SV replacement time of about 4 days and using a prior-art sparing approach, over 230 kg of propellant is required. This assumes a Delta-V of about 600 meters/second and a spacecraft dry mass of approximately 840 kg. The worst-case 360 degree phase change necessary for SV replacement may be accomplished within about 4 days starting from a sparing orbit with a drift rate of about 90 degree/day relative to the mission orbit. Unfortunately, accommodating this large amount of propellant increases the SV size and mass, increasing the number of launch vehicles needed to populate the constellation, and increasing the launch cost.